This invention relates to a fixing roll used in equipment such as an electrographic copying machine, a facsimile machine, or a printer which utilizes an electrographic process.
In a general electrographic process, a toner image is first formed on a photo-sensitive member and is then transferred by a transferring unit onto a recording medium such as a recording sheet. Thereafter, when necessary, the toner image thus transferred is fixed onto the recording medium, to provide a desired copy.
Methods known in the art for fixing toner images are the methods of heat-type fixing, pressure-type fixing, and solvent-type fixing. The heat-type fixing method is extensively employed. In the heat-type fixing method, toner is melted by heat so that it sticks onto a recording medium. The heat-type fixing method is further divided into a hot air-type fixing method, a heat roll-type fixing method, and others. Of these, the heat roll-type fixing method is most generally employed. The heat roll-type fixing method uses a thermal fixing roll 1, illustrated in cross-section in the FIGURE and a pressure roll 4 which is pushed against the roll 1 under a certain pressure. The thermal fixing roll 1 comprises a cylindrical rough core 2, a heater H placed within the core 2, and a heat-resistant layer 3 formed on the outer wall of the core 2. A sheet P having a toner image T is passed through the rolls 1 and 4, so that the toner image T is fixed onto the sheet P by the conductive heat of the thermal fixing roll 4.
The heat-resistant layer (or heat-resistant elastic layer) 3 of the thermal fixing roll 1 must be resistant not only against the effects of heat but also against pressure, and should be made of a material to which toner barely adheres. Generally, the heat-resistant layer 3 is made of fluororesin (polytetrafluoroethylene resin), HTV (high temperature vulcanization) silicone rubber, or RTV (room temperature vulcanization) silicone rubber.
When the heat-resistant layer 3 is formed by applying fluororesin to the core 2, its thickness is generally of the order to several microns (.mu.m) to several tens of microns. When it is made of silicone rubber, its thickness is about 300 .mu.m. Thus, a heat-resistant layer of silicone rubber has substantial thickness. The thermal conductivity of silicone rubber is 4.times.10.sup.4 cal/cm sec .degree.C., which is appproximately equal to that of an asbestos sheet or cork plate. Accordingly, it takes a relatively long time to increase the surface temperature of the thermal fixing roll 1 to a predetermined value. As a result the thermal fixing roll having a heat-resistant layer of silicone rubber is not practical. Furthermore, since the thermal conductivity of silicone rubber is low, it takes a long time to restore the surface temperature of the roll which has decreased because of the passage of a sheet P. Accordingly, if the thermal fixing roll 1 is used continuously with a heater H of low heat rating, the surface temperature of the roll 1 is decreased to the extent that the toner image is not satisfactorily fixed.
In order to overcome this difficulty, metal powder or metal oxide powder is often mixed with the silicone rubber, to increase the thermal conductivity of the heat-resistant layer 3. However, if a fixing unit employs a heat-resistant elastic layer 3 which is made of a mixture of silicone rubber and metal powder or metal oxide powder (such as alumina or iron oxide red), the following difficulty arises. When the fixing unit is repeatedly used, toner is accumulated on the surface of the thermal fixing roll. As a result, the toner image on a sheet passing the rolls is transferred onto the thermal fixing roll and is transferred therefrom onto the following sheet. That is, a so-called "offset phenomenon" is liable to take place, and the service life of the thermal fixing roll 1 is thereby reduced. A toner offset preventing solution is sometimes used to overcome this problem.